-LRB- CNN -RRB- -- At least 150 people drowned when a boat leaving Libya capsized off the Tunisian coast this week , the U.N. refugee agency said Friday .

Scores more were still missing as a rescue operation by Tunisian authorities continued . But at least 578 people survived the sinking Wednesday , according to the U.N. High Commissioner for Refugees .

On Thursday , 195 survivors were transferred to a humanitarian aid camp close to the Tunisia-Libya border , according to a statement from the refugee agency .

An additional 383 survivors were expected to be taken to the same or nearby camps for help .

The overcrowded boat , which set sail May 28 from Tripoli , was headed for the Italian island of Lampedusa carrying about 850 people . The passengers were mostly migrants from West Africa , Pakistan and Bangladesh who were fleeing fighting in Libya , the refugee agency said in a statement .

The agency said some survivors reported the boat was manned by people with little or no maritime experience .

Soon after the boat left Tripoli , it ran into difficulties and by Tuesday , the passengers ran out of food and water .

The boat ran aground Wednesday near the Kerkennah Islands , about 300 kilometers -LRB- about 186 miles -RRB- northwest of Tripoli in the Mediterranean Sea , the refugee agency reported .

`` Desperate passengers rushed to one side , seeking rescue by the Tunisian coast guard and fishing boats that had approached the vessel , '' and the boat capsized , the agency said .

`` This appears to be one of the worst and the deadliest incidents in the Mediterranean so far this year , '' said Adrian Edwards , a spokesman for the refugee agency .

Lampedusa , the closest Italian island to Africa , has become a destination for tens of thousands of refugees seeking to enter Europe .

More than 30,000 migrants and refugees from Tunisia and Libya have risked the dangerous journey to Lampedusa .

At one point , the population of migrants vastly outnumbered the tiny population of Lampedusa , which numbers less than 6,000 .

`` Boats loaded with migrants fleeing the ongoing conflict in Libya have been making the journey to Italy and Malta over recent months , sometimes with tragic consequences , '' the refugee agency said in its statement . `` Just last month , hundreds died as a vessel carrying about 600 people broke up shortly after departing Tripoli . ''

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U.N. refugee agency : 578 people survive the sinking

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Thousands of refugees fleeing the conflict in Libya try to get to Europe

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An overcrowded boat with about 850 people aboard left Tripoli for Italy last weekend

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The boat capsized in the Mediterranean Sea on Wednesday